The overall goal of this project is to exploit the RNA interference (RNAi) mechanism in order to develop therapeutics to treat human disease. Because the RNAi mechanism is extraordinarily potent at reducing expression of a specific gene, it offers an opportunity to rationally design drugs targeted to individual genes in a disease implicated pathway. While this paradigm has largely been realized in cell culture using cationic lipids to introduce substrates for the RNAi pathway into mammalian cells, the widespread application of RNAi technology to gene downregulation in animals remains elusive. This is largely due to the poor pharmacokinetic (PK) properties of RNAi based drugs such as small interfering RNAs (siRNAs). We believe that that unmodified siRNA will fail as systemically administered drugs due to the poor PK properties of RNA, in particular insufficient stability in biological fluids such as blood, and suboptimal penetration into cells and uptake by tissues. We intend overcome these limitations by "employing our knowledge of the structure-activity relationships (SAR) of siRNAs to design analogs which have the potential to exhibit improved PK properties, yet maintain the high intrinsic potency of siRNA in reducing target gene expression. We will then evaluate these improved siRNA designs in preliminary pharmacokinetic and pharmacology studies in order to assess their ability to serve as leads for the development of robust RNAi based therapeutics. The deliverable from the first phase this project is one or more chemically modified siRNA constructs which show improved pharmacokinetic properties in animals relative to unmodified siRNAs. This work will be expanded in the second phase of the project to provide optimized motifs which have superior pharmacokinetics and show robust pharmacology in animal models of human disease. Because of our focus on a novel target relevant to cancer chemotherapy, we will also generate therapeutic leads appropriate for pre-clinical drug development activities. The deliverable from the project will be optimized siRNA constructs which are widely applicable to the modulation of gene expression in animals. [unreadable] [unreadable]